1. Field of the Invention
The present invention relates generally to a method of treating a surface of a semiconductor substrate and, more particularly, to a method of cleaning the surface of the semiconductor substrate without a damaged layer left in the surface thereof. The invention further relates to a method of manufacturing a semiconductor device including such a method of treating the semiconductor substrate surface.
2. Description of the Background Art
A surface of a semiconductor substrate, e.g., a silicon substrate must be cleaned in order to manufacture a highly reliable semiconductor device.
FIGS. 7A-7D are cross-sectional views showing a conventional treating method of a semiconductor substrate surface. (See 1985 Dry Process Symposium.)
Referring to FIG. 7A, a native oxide film 2 is usually formed on a surface of a silicon substrate 1. This native oxide film 2 is removed by a reactive ion etching employing a CHF.sub.3 gas. At this time, with reference to FIG. 7B, a fluorocarbon layer (a polymer layer of CF.sub.x) 3 is formed on the surface of silicon substrate 1. Since the surface of silicon substrate 1 is subjected to plasma irradiation, a surface damaged layer 31 is formed in the surface of silicon substrate 1.
Referring to FIG. 7C, the surface of silicon substrate 1 is irradiated with ultraviolet rays in a C1.sub.2 gas atmosphere in order to remove fluorocarbon layer 3. Thus, with reference to FIG. 7D, fluorocarbon layer 3 adhering to the surface of silicon substrate 1 is removed.
In this method, however, although fluorocarbon layer 3 can be removed without native oxide film 2 left, there is a disadvantage that surface damaged layer 31 formed in the surface of silicon substrate 1 cannot be removed.
FIGS. 8A-8D are cross-sectional views showing another conventional example of a treating method of a semiconductor substrate surface.
Referring to FIG. 8A, a native oxide film 2 formed on the surface of a silicon substrate 1 is removed by a reactive ion etching employing a CHF.sub.3 gas or a mixed gas of C.sub.m F.sub.n, H.sub.2 and the like.
At this time, with reference to FIG. 8B, a fluorocarbon layer (a polymer layer of CF.sub.x) 3 is formed on the surface of silicon substrate 1. Since the surface of silicon substrate 1 is subjected to plasma irradiation, a surface damaged layer 31 is formed in the surface of silicon substrate 1. Referring to FIGS. 8C and 8D, the surface of silicon substrate 1 is lightly etched by afterglow discharge employing a mixed gas of CF.sub.4 and O.sub.2 in order to remove fluorocarbon layer 3 and surface damaged layer 31. Conventionally, the surface of the silicon substrate is treated by this method, thereby decreasing an electric resistance of the substrate.
In addition, the foregoing method is applied not only to the step of removing native oxide film 2 but also to a single step for manufacturing a transistor, i.e., the step of selectively etching a silicon oxide film covering the surface of the substrate in order to form sidewall spacers on sidewalls of a gate.
However, with reference to FIG. 8D, the surface treatment method of the above-described latter conventional example has a disadvantage that fluorine remains in the form of an SiF.sub.x layer 4 in the surface of silicon substrate 1.
If such an SiF.sub.x layer 4 exists in the surface of silicon substrate 1, the following problems arise. More specifically, with reference to FIG. 9A, there is a problem that fluorine causes abnormal acceleration of an oxidation reaction in the step of forming a gate oxide film 41 for forming, for example, a transistor, on a silicon substrate 1. That is, when gate oxide film 41 having a film thickness d shown in FIG. 9A is formed on silicon substrate 1, there is a problem that a gate oxide film 41 having an extremely large film thickness d.sub.2 (d.sub.2 &gt;d.sub.1) is obtained as shown in FIG. 9B. If the film thickness of gate oxide film 41 increases, an interface state of the gate oxide film becomes higher, leading to a decrease in gate junction voltage strength. The decrease in gate junction voltage strength causes a degradation in electrical characteristics, resulting in a lower reliability of the semiconductor device.